Characterization of amino acid transport systems in human placental basal membrane vesicles.
The amino acid transport systems have been characterized in basal membrane vesicles prepared from human full-term placental syncytiotrophoblasts. Transport of amino acids across basal membranes occurred via passive diffusion and Na(+)-independent and Na(+)-dependent carrier-mediated systems. Passive diffusion was responsible for a substantial fraction of transport. L-Glutamate and alpha-(methylamino)isobutyrate were transported only Na(+)-independently, while the transport of L-alanine was dependent solely on an Na+ gradient from the outside to the inside of the vesicles. L-Methionine, L-leucine, glycine and L-proline transport were supported by both Na(+)-independent and Na(+)-dependent systems. L-Lysine transport was decreased in the presence of cations, an inwardly directed Na+ gradient was much more effective than a K+ gradient at slowing L-lysine transport. A cross-inhibition analysis of these amino acids indicates that at least three Na(+)-independent and five Na(+)-dependent carrier-mediated systems exist in the human placental syncytiotrophoblast basal membranes. One Na(+)-independent system interacts with all substrates tested. Another Na(+)-independent system carries glycine, L-methionine, L-leucine and L-lysine; it is sensitive to L-glutamate, but not to L-proline or alpha-(methylamino)isobutyrate. The third system is selective for L-lysine, which is inhibited by L-methionine, glycine and L-leucine, but inaccessible to L-glutamate, L-proline and alpha-(methylamino)isobutyrate. One Na(+)-dependent system carries L-alanine, glycine, L-methionine and L-leucine, and it is sensitive to L-proline. The second system mediates transport of L-alanine, glycine, L-methionine and L-proline, but is not sensitive to L-leucine. The third system carries L-alanine, glycine and L-proline, and is inaccessible to L-methionine and L-leucine. The fourth system is responsible for L-methionine and L-leucine; it is sensitive to L-alanine and glycine, but not to L-proline. The fifth system is selective for L-proline.
Kudo
Y
Y
Department of Human Anatomy, University of Oxford, U.K.
Boyd
C A
CA
D000596:Amino Acids / Q000378:metabolism* / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism*; D004058:Diffusion; D005260:Female; D006801:Humans; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D012964:Sodium / Q000494:pharmacology; D014327:Trophoblasts / Q000378:metabolism*
D000596:Amino Acids; D012964:Sodium